Short Answer:

An optical tachometer works on the principle of light reflection and interruption. It uses a light source, usually a laser or LED, and a photodetector to sense the rotation of an object. A reflective mark or slot is placed on the rotating shaft, and every time this mark passes through the light beam, it reflects or blocks the light.

The photodetector counts these reflections or interruptions and generates electrical pulses. The number of pulses per second is then used to calculate the rotational speed (RPM) of the object. It is a non-contact, accurate, and safe method for speed measurement.

Detailed Explanation:

Principle of optical tachometer

An optical tachometer is a non-contact type speed measuring device that determines the rotational speed of a shaft or disk using light. Unlike mechanical tachometers, it does not touch the rotating surface, making it ideal for high-speed or sensitive applications where physical contact could cause damage or wear.

The core working principle is based on the detection of reflected or interrupted light and counting how often this occurs within a set period. This gives a reliable way to calculate speed in revolutions per minute (RPM).

Main Components:

1. Light Source:
   Typically an LED or laser that emits a focused beam of light toward the rotating surface.
2. Reflective Surface or Slot:
   A reflective tape or bright marker is attached to the rotating part (or a cut-out slot in some designs).
3. Photodetector (Sensor):
   Positioned to receive the reflected light from the marker or detect when light is interrupted.
4. Electronic Circuit and Display:
   The received signal is processed to count the pulses and display the RPM value.

Working Steps:

1. Setup:
   A small reflective strip is attached to the surface of a rotating shaft or disk. The tachometer is placed at a short distance pointing at this surface.
2. Light Emission:
   The tachometer sends out a continuous beam of light (laser or infrared) toward the rotating surface.
3. Light Reflection or Interruption:
   As the object rotates, the reflective mark passes in front of the light beam once per revolution.
   • If using a reflective type, the light is reflected back only when the reflective strip passes.
   • If using a beam-break type, the light beam is blocked momentarily by a rotating blade or slot.
4. Signal Generation:
   Each time the light is reflected or blocked, the photodetector senses a change and generates an electrical pulse.
5. Speed Calculation:
   The circuit counts the number of pulses received in one second and multiplies by 60 to get the RPM.
   For example, if 30 pulses are counted in one second, the speed is:
   30 × 60 = 1800 RPM

Advantages:

• Non-contact measurement – no friction or wear
• Safe and easy to use – ideal for fast or delicate machines
• High accuracy and quick response
• No mechanical parts – low maintenance
• Can measure both low and high speeds

Limitations:

• Needs clear access to the rotating part
• Affected by ambient light, dust, or dirt on the surface
• Requires power supply for electronics
• Reflective tape must be properly installed and aligned

Applications:

• Industrial motors and pumps
• Engine and turbine speed monitoring
• Conveyor systems
• Laboratory testing equipment
• Robotics and automated machinery

Conclusion:

An optical tachometer works on the principle of detecting reflected or interrupted light from a rotating object. By counting these light pulses, it accurately calculates the rotation speed without any physical contact. This makes it a preferred choice in many industries where speed measurement must be precise, safe, and free from mechanical interference.